Final answer:
Dr. Josef Mengele conducted selections at Auschwitz concentration camp to determine those fit for labor and those to be executed, while simultaneously selecting subjects for his medical experiments. The Einsatzgruppen carried out mass shootings, leading to the use of gas vans and extermination camps for systematic mass murder.
Step-by-step explanation:
When men marched before Dr. Josef Mengele, they were subjected to a selection process. Mengele was known as the 'Angel of Death' for his role in determining the fate of prisoners at Auschwitz concentration camp during the Holocaust. He would decide who was fit for forced labor and who would be sent immediately to the gas chambers for execution. His purpose was to eliminate those deemed unfit for work and to select subjects for his inhumane medical experiments that focused on genetic research and pseudoscientific racial theories.
The Einsatzgruppen, mobile killing units, followed the German army's invasion into the Soviet Union in 1941 and were responsible for mass shootings of Jewish communities, Roma, and political dissidents. Subsequent psychological issues experienced by soldiers from direct involvement in these atrocities led to the development of gas vans and, eventually, the systematic mass murder in extermination camps such as Auschwitz, where Mengele operated.